Radio receiving system



March 20, 1934. A. A. SCHOEN Er AL RADIO RECEIVING SYSTEM Filed Nov. 13,1929 INF INVENTORS M. scnoeu m BY NW.

ATTORN EY Cal Patented Mar. 20, 1934 UNITED STATES PATENT oFicE Blyth,Brooklyn, N. Y.

, assignors to Radio Corporation of America, a corporation of DelawareApplication November 13, 1929, Serial No. 406,756

18 Claims.

This invention relates to radio reception and has as its prime objectthe provision of a novel method and means for reducing image frequencyresponse in a superheterodyne receiver.

It often occurs, when utilizing a superheterodyne receiver, that whenthe local oscillator thereof is tuned to obtain beat frequency energywith a desired station, another station is heard, which, of course, isundesirable. This interference is due to the fact that the undesiredstation produces the same beat frequency with the local oscillator thatthe desired station does; and, in the past was avoided by manuallytuning the oscillator to another frequency either above or below itsformer frequency depending upon its former relationship with the desiredstation. However, to most radio receiver operators, such procedure ishighly bothersome as it involves additional careful manipulation of theoscillator tuning dials.

Accordingly, another object of this invention is to provide asuperheterodyne receiver wherein the shift in oscillator frequency toreduce image frequency response is accomplished automatically.

In addition, we have found, depending upon 'the location in which thereceiver is placed, that image frequency response is present only over aportion of the range of frequencies to which the receiver is responsive.Therefore, if the receiver is taken from one locality to another, the'range or portion of the scale on which image frequency response occurs,changes both in magnitude and relative angular position; or in otherwords, in frequency band width and in position on the frequencyspectrum.

Hence, to provide an automatic change-over device for shiftingoscillator frequency variable in width and position is a further objectof this invention.

Briefly, according to a preferred embodiment of our invention, theforegoing objects are carried out by inserting a reactance in thetunable control circuit of the superheterodyne local oscillator by meansof a switching arrangement automatically operable during themanipulation of 'the variable element of the tunable circuit whichcontrols the frequency of the local oscillator. As the switchingmechanism is in the form of a commutator mounted upon the elementvarying the frequency of the local oscillator generator, the angularposition at which change-over occurs can be readily controlled. By meansof a variable contact, the portion or width of the frequency band inwhich frequency shift is accomplished, is made variable.

The features of this invention which we believe characterize it arepointed out with particularity in the appended claims; however, ourinvention may best be understood both as to its mode of operation andconstruction by referring to the accompanying drawing which, of course,is given solely by way of illustration and not by way of limitation.

Referring to it,

Figure 1 is a diagrammatic illustration of a superheterodyne receiverembodying the principles of our invention; V

Figure 2 illustrates a mechanism for providing variable frequency rangechange-over both in magnitude and position; and,

Figures 3 and 4 indicate alternative means for causing a predeterminedfrequency shift in an oscillation generator. 7,

Turning to Figure 1, numeral 2 indicates an antenna which may be a loopor any other desired type of antenna feeding energy into an untunedamplifier, comprising preferably a screen grid vacuum tube 4, for thepurpose of preventing radiation from subsequent apparatus in thereceiver over antenna 2. The output of amplifier 4 is fed into anoscillating detector OD which in turn feeds beat frequency energy into atuned intermediate frequency amplifier 6 tuned to a constant frequency.The output of the intermediate frequency amplifier 6 is fed into adetector or rectifier 8, rectified, and thence fed if desired, into arelatively low frequency amplifier 10 and thence translated by anysuitable means such as a telephone 12. In order to control and vary thefrequency of oscillation of the oscillating detector or local generatorof oscillations a tunable circuit 14 is provided comprising an inductivereactance 16 and a capacitivereactance 18 the latter having a straightline frequency characteristic and having fixed stator plates 20 andgrounded rotor plates 22.

In order to translate signals from a desired station, the rotor platesof the condenser in tunable circuit 14 are varied by means of knob 24and shaft 26 until the local oscillation generator produces energy ofcorrect frequency to give a beat note frequency with the desired stationcorresponding to the tuning of intermediate frequency amplifier 6.

As already indicated, over a given range of frefrequency from afrequency higher than the desired station frequency to one below. Overthat range of frequencies, by means of the commutator or switchingmechanism X, a fixed reactance 28 is connected into circuit with thetunable circuit. Reactance 28 in the embodiment shown is a condenser offixed value. As the switching arrangement is fixed to the control shaft26 its action becomes, of course, automatic during the troublesome rangewhere image phenomena occur.

The switching arrangement is shown in greater detail in Figure 2together with the means for making variable the magnitude and positionof the range of frequencies at which change-over occurs. Morespecifically, the switching or change-over mechanism comprises a disk ofinsulating material 30 having an insulating hub 32. By means of a setscrew 34 the disk and hub can be placed at any desired angular positionon shaft 26. In this manner the position in the frequency spectrum atwhich frequency shift over occurs is readily varied.

Hub 32 carries a metallic commutator segment 36 and disk 30 carries ametallic segment 38. Both segments are connected together by means of ametallic connecting piece 40 contacting with the under surface ofsegment 38 and thence over one surface of disk 30 to contact with theunder surface of segment 36 at 42. Segment 36 is grounded by means of acontacting element 44.

One side of condenser 28 is connected to stator plates of condenser 20and the other side iscon nected to ground through a contacting member 46and segment 38 only when member 46 is contacting with segment 38.Contacting element 46 is carried by means of screw clamps 48, 50 on aninsulating support 52 and, accordingly, may be varied in position alongsegment 38 as shown by the dotted lines. This variation, of course,gives the magnitude or range width at which it is desired that frequencychange-over of the oscillator be made.

Another way of causing shift of oscillator frequency, from frequenciesabove desired stations to that below the stations for reducing imagefrequency response,'is shown in Figure 3 wherein to the rotor plates 22of the oscillator condenser,

areas 54 are added so that for a given range the desired shift infrequency takes place.

In the event that it is desired to shift the o."- cillator frequency inthe reverse manner, apparatus as shown in Figure 4 may be resorted toWherein'by means of the cut away portions 56, on

the oscillator rotor condenser plates 22, the

change-over will be from a lower to a higher fre fractional portion onlyof its range.

1 2. An oscillation generator comprising an electron discharge tubeincluding a tunable input circuit for controlling the frequency thereof,said tunable circuit comprising a fixed inductance and a variablecapacity having a straight line frequency characteristic and beingcontinuously variable to alter the frequency of the tunable circuit overa given range, and means for automatically connecting a fixed capacityto said variable capacity over a portion only of its range.

3. In a receiver, a signal amplifier circuit, a local oscillator stageincluding a tube provided with a tunable input circuit, an amplifiertuned to a predetermined beat frequency, said tunable input circuitincluding a pair of reactances of opposite sign, means for varying oneof said reactances to produce said beat frequency with amplifieddifferent signals, and additional means mechanically operative with saidvarying means, and including a fixed reactance, for producing said beatfrequency with a desired signal without image interference.

4. In a superheterodyne receiver, a signal amplifier circuit, includinga screen grid tube, a local oscillator stage including a tube providedwith a tunable input circuit, an amplifier tuned to a predetermined beatfrequency, said tunable input circuit including a pair of reactances ofopposite sign, means for varying one of said reactances to produce saidbeat frequency with amplified different signals, and additional meansmechanically operative with said varying means, and including a fixedreactance of the same sign as the variable reactance, for producing saidbeat frequency with a desired signal without image interference.

5. In a receiver, a signal amplifier circuit, a local oscillator stageincluding a tube provided with a tunable input circuit, an amplifiertuned to a predetermined beat frequency, said tunable input circuitincluding a pair of reactances of opposite sign, means for varying oneof said reactances to produce said beat frequency with amplifieddifferent signals, and additional means mechanically operative with saidvarying means, and including a fixed capacity, for producing said beatfnequency with a desired signal without image interference.

6. In a receiver, a signal amplifier circuit, a local oscillator stageincluding a tube provided with a tunable input circuit, an amplifiertuned to a predetermined beat frequency, said tunable input circuitincluding an inductance and a capacity, means for varying said capacityto produce said beat frequency with amplified different signals, andadditional means mechanically operative with said varying means, andincluding a fixed reactance, for producing said beat frequency with adesired signal without image interference.

'7. In a receiver, a signal amplifier circuit, a local oscillator stageincluding a tube provided with a tunable input circuit, an amplifiertuned to a predetermined beat frequency, said tunable input circuitincluding an inductance and a capacity, means for varying said capacityto produce said beat frequency with amplified different signals, andadditional means mechanically operative with said varying means, andincluding a fixed capacity for producing said beat frequency, with adesired signal without image interference.

8. In a superheterodyne receiver, a signal amfier circuit, including ascreen grid tube, a local oscillator stage including a tube providedwith a tunable input circuit, an amplifier tuned to a predetermined beatfrequency, said tunable input circuit including an inductance and acapacity,

means for varying said capacity to produce said beat frequency withamplified different signals. and additional means mechanically andsimultaneously operative with said varying means, and including a fixedcondensive reactance, for producing said beat frequency with a desiredsignal without image interference.

9. In a superheterodyne receiver, a signal amplifier circuit, includinga screen grid tube, a local oscillator stage including a tube providedwith a tunable input circuit, an amplifier tuned to a predetermined beatfrequency, said tunable input circuit including an inductance and acapacity, means for varying said capacity to produce said beat frequencywith amplified different signals, and additional means mechanically andsimultaneously operative with said varying means, and including a fixedcapacity for producing said beat frequency with a desired signal withoutimage interference.

10. In a superheterodyne receiver an untuned radio frequency amplifierincluding a screen grid tube, a local oscillator including an electrondischarge tube provided with a tunable input circuit, an intermediatefrequency amplifier, said oscillator input circuit including a fixedinductance and a capacity, means for varying said capacity to producesaid intermediate frequency with amplified different signals, andadditional means mechanically and simultaneously operative with saidcapacity varying means, and including a fixed reactanoe, for connectingthe latter to said capacity to produce said intermediate frequency witha desired signal without image interference.

11. In a superheterodyne receiver an untuned radio frequency amplifierincluding a screen grid tube, a local oscillator including an electrondischarge tube provided with a tunable input circuit, an intermediatefrequency amplifier, said oscil- 'lator input circuit including a fixedinductance and a variable capacity having a straight line frequencycharacteristic, means for varying said capacity to produce saidintermediate frequency with amplified different signals, and additionalmeans operative with said capacity varying means, and including a fixedreactance, for connecting the latter to said capacity to produce saidintermediate frequency with a desired signal without image interference.

12. In a superheterodyne receiver an untuned radio frequency amplifierincluding a screen grid tube, a local oscillator including an electrondischarge tube provided with a tunable input circuit, an intermediatefrequency amplifier, said oscillator input circuit including a fixedinductance and a variable capacity having a straight line frequencycharacteristic, means for varying said capacity to produce saidintermediate frequency with amplified different signals, and additionalmeans operative with said capacity varying means, and including a fixedcapacity, for connecting the latter to said capacity to produce saidintermediate frequency with a desired signal Without image interference.

13. A local oscillator, adapted for use in conjunction with a source ofsignals of a desired frequency range and a beat frequency utilizationcircuit, comprising a tube having a resonant input circuit adapted to becoupled to the source and an output circuit adapted to be coupled tosaid beat circuit, a variable tuning condenser in said input circuithaving a grounded rotor element, a fixed condenser having one sidethereof connected to the stator element of the tuning condenser, andgrounded means, operative with said rotor element through a portion ofsaid range, adapted to connect the other side of said fixed condenser tosaid rotor element.

14. A local oscillator, adapted for use in a superheterodyne receiverprovided with a signal input circuit and an intermediate frequencyamplifier, comprising a tube having a tunable input circuit including avariable condenser for adjusting the oscillator through a range ofoscillation frequencies adapted to heterodyne with a desired range ofsignal frequencies separatedfrom said oscillation frequencies by saidintermediate frequency, and means, movable with the variable element ofthe condenser, for changing the effective capacity of said tunable inputcircuit through a portion of said oscillation frequency range to shiftthe oscillation frequency values in said portion to values capable ofproducing said intermediate frequency with the desired signals but notwith undesired signal image frequencies.

15. A local oscillator, adapted for use in a superheterodyne receiverprovided with a signal input circuit and an intermediate frequencyamplifier, comprising a tube having a tunable input circuit including avariable condenser for adjusting the oscillator through a range ofoscillation frequencies adapted to heterodyne with a desired range ofsignal frequencies separated from said oscillation frequencies by saidintermediate frequency, and means, movable with the variable element ofthe condenser, for increasing the effective capacity of said tunableinput circuit through a portion of said oscillation frequency range toshift the oscillation frequency values in said portion to values capableof producing said intermediate frequency with the desired signals butnot with undesired signal image frequencies.

16. A local oscillator, adapted for use in a superhetercdyne receiverprovided with a signal input circuit and an intermediate frequencyamplifier, comprising a tube having a tunable input circuit including avariable condenser for adjusting the oscillator through a range ofoscillation frequencies adapted to heterodyne with a desired range ofsignal frequencies separated from said oscillation frequencies by saidintermediate frequency, and means, movable with the variable element ofthe condenser, for decreasing the effective capacity of said tunableinput circuit through a portion of said oscillation frequency range toshift the oscillation frequency values in said portion to values capableof producing said intermediate frequency with the desired signals butnot with undesired signal image frequencies.

17. A local oscillator, adapted for use in a superheterodyne receiverprovided with a signal input circuit and an intermediate frequencyamplifier, comprising a tube having a tunable input circuit including avariable condenser for adjusting the oscillator through a range ofoscillation frequencies adapted to heterodyne with a desired range ofsignal frequencies separated from said oscillation frequencies by saidintermediate frequency, and means, movable with the variable element ofthe condenser, for changing the effective capacity of said tunable inputcircuit through a portion of said oscillation frequency range to shiftthe oscillation frequency values in said portion to values capable ofproducing said intermediate frequency with the desired signals but notwith undesired signal image frequencies and additional means for varyingthe magnitude of said portion and the position thereof in saidoscillation frequency range.

18. An oscillation generator comprising an electron discharge tubehaving a tunable input condenser and means for connecting the otherplate of said fixed condenser to the said rotor plates, said meanscomprising a metallic segment, an insulating disc on which said segmentis mounted said disc being loosely mounted on said shaft and means forretaining said disc in any desired angular position on said shaft.

ARTHUR A. SCHOEN. ARTHUR A. BLYTI-Ir

